1. Field of the Invention
The present invention relates to a chip piezoelectric filter that has a circuit configuration wherein a plurality of piezoelectric filter sections are electrically connected. More specifically, the present invention relates to a chip piezoelectric filter that has a configuration wherein each of a plurality of piezoelectric substrates has an energy-trap piezoelectric filter section provided thereon.
2. Description of the Related Art
In an intermediate-frequency stage filter of a mobile communication apparatus, such as a portable telephone, a piezoelectric filter of an energy-trap type is used. As with other electronic components, the piezoelectric filter of this type is also required to be configured as a chip component to enable surface-mounting.
An example of the piezoelectric filter of this type is disclosed in Japanese Unexamined Patent Application Publication No. 10-335976. FIGS. 7 and 8 show the conventional piezoelectric filter.
A piezoelectric filter 50 has a first piezoelectric substrate 51 and a second piezoelectric substrate 52 that are stacked on each other and separated from each other via a spacer 58. On each of the first and second piezoelectric substrate 51 and 52, an energy-trap piezoelectric filter section is provided. Also, a pair of resonant electrodes 53a and 53b is provided on the upper surface of the first piezoelectric substrate 51, and a common electrode (not shown) is arranged opposite to the resonant electrodes 53a and 53b with respect to the front and reverse surfaces thereof. The resonant electrodes 53a and 53b and the common electrode provide the energy-trap piezoelectric filter section that vibrates in a expansion oscillation mode.
On the upper surface of the second piezoelectric substrate 52, a common electrode 53c is provided. Similar to the first piezoelectric substrate 51, a pair of resonant electrodes (not shown) are arranged to oppose the common electrode 53c with respect to the front and reverse surfaces thereof. The pair of electrodes and the common electrode 53c define a second energy-trap piezoelectric filter section.
An opening 58a is provided in the spacer 58 to prevent interference of vibration of the energy-trap piezoelectric filter section. Also, confining substrates 60 and 61 are, respectively, stacked on outer main surfaces of the first and second piezoelectric substrates 51 and 52 via spacers 57 and 59.
The spacers 57 and 59 have an opening 57a and an opening 59a, respectively. The openings 57a and 59a are arranged to define cavities that prevent interference of vibrations of the first and second piezoelectric filter sections of the energy-trap type. Specifically, as shown in FIG. 8, a first cavity X, a second cavity Y, and a third cavity Z are provided. The first cavity X is provided between the first and second piezoelectric substrates 1 and 2. The second cavity Y is provided with the spacer 57 on an upper portion of the first piezoelectric substrate 51. The third cavity Z is provided with the spacer 59 in a lower portion of the second piezoelectric substrate 52. The sizes of the cavities X, Y, and Z are the same.
The described energy-trap piezoelectric filter is used as a bandpass filter that is required to reduce unnecessary spurious components of attenuation-frequency characteristics.
However, the energy-trap piezoelectric filter frequently produces large spurious components in areas close to passbands.